From the Castle Rock trailhead, trail starts as an overgrown doubletrack trail and continues as such for approximately .2 miles. It then dumps out at a dead end on a gravel road. Continue on gravel road for ~300 feet and trail will once again be on the left, but this time as pristine singletrack for 2.7 miles. Echo trail is rarely used, but has some great features that make it completely scenic and worth riding. It is an undulating trail with beautiful, CCC built stone switchbacks that crosses over water streams complete with beautiful mossy rocks. Includes some punchy climbs and rad descents. Little, very natural side hits can be found on occasion if you're looking and willing to shred through some salal. Trail drains well due to rocky soil and is, despite not being ridden much, very easy to navigate and clear of most brush and thusly in great condition.
Personal favorite way to ride this is to park/take bus to King-Castle Trailhead and ride up K-C to where it meets the Castle Rock trail, I then head down Castle Rock (lower, less used section) and ride that to the end. I will then proceed to ride up the very, very last portion of of O'Leary to the first (or last, depending on direction) road crossing and then descend the road approximately 500 feet to the start of Echo trail (on left, marked with sign). I do an OAB on this route.
Alternative way would be to include it as a finisher, or start to an O'Leary ride by parking at the end of O'Leary, then riding Echo to the 1993 road (East fork parking lot just below) and then riding up the 1993 to the top of O'Leary (or Ollalie).